1. Field of the Invention
The present invention relates to an improvement in a focusing state detection system which detects a focusing state by a relative positional relationship of two images of an object.
2. Related Background Art
In a prior art focus state detection apparatus for a camera, an exit pupil of an imaging lens is divided into two pupil areas and relative positional displacement of two images formed by light fluxes transmitted through the pupil areas are observed to determine an in-focus state. For example, Japanese Laid-open Patent Application Nos. 118019/1980 and 155331/1985 disclose a secondary focusing system in which a spatial image formed on a predetermined focus plane (corresponding to a film plane) by two secondary focusing optical systems is guided to two sensor planes so that relative positional displacement of the two images is detected.
The secondary focusing type focusing state detection apparatus is shown in FIG. 4. A field lens 3 is arranged coaxially with an optical axis 2 of an imaging lens 1 whose focusing state is to be detected. Two secondary focusing lenses 4a and 4b are arranged behind the field lens 3 symmetrically with respect to the optical axis 2. Photo-electric conversion element arrays 5a and 5b are arranged behind the lenses 4a and 4b. Diaphragms 6a and 6b are arranged in the vicinity of the secondary focusing lenses 4a and 4b. The field lens 3 essentially focuses an exit pupil of the imaging lens 1 onto pupil planes of the two secondary focusing lenses 4a and 4b. As a result, light fluxes applied to the secondary focusing lenses 4a and 4b correspond to those light fluxes which are emitted from non-overlapping equi-space areas on the exit pupil plane of the imaging lens 1, corresponding to the secondary focusing lenses 4a and 4b. When a spatial image formed in a vicinity of the field lens 3 is refocused on the planes of the photo-electric conversion element arrays 5a and 5b by the secondary focusing lenses 4a and 4b , the positions of the two images on the photo-electric conversion element arrays 5a and 5b change in accordance with the displacement of the spatial image along the optical axis. FIG. 5 shows this. In FIG. 5A which shows an in-focus state, the two images are positioned at the centers of the photo-electric conversion element arrays 5a and 5b, in FIG. 5B which shows a near-focus state, the two images are moved away from the optical axis 2, and in FIG. 5C which shows a far-focus state, the two images are moved toward the optical axis 2. This image intensity distribution is photo-electrically converted and the converted electrical signal is processed to detect a relative positional deviation of the two images. In this manner, the focus state of the imaging lens 1 can be detected.
Methods for processing the photo-electrically converted signal from the photo-electric conversion element arrays 5a and 5b are disclosed in Japanese Laid-open Patent Application No. 142306/1983 (U.S. Pat. No. 4,559,446) and U.S. Pat. No. 4,333,007. Specifically, the following formula is operated for k.sub.1 .ltoreq.k.ltoreq.k.sub.2 (for example, k.sub.1 =-N/2, k.sub.2 =N/2). ##EQU1## where N is the number of photo-electric conversion elements of the photo-electric conversion element array 5a or 5b, A(i) and B(i) are image signals from the i-th elements of the photo-electric conversion element arrays 5a and 5b, and M is the number of pixels processed (M=N-.vertline.k.vertline.-1). A(i).quadrature.B(j) is an operator for A(i) and B(j). For example, EQU A(i).quadrature.B(j)=.vertline.A(i)-B(j).vertline. (2) EQU A(i).quadrature.B(j)=.vertline.A(i)-B(j).vertline..sup.n ( 3) EQU A(i).quadrature.B(j)=max[A(i), B(j)] (4) EQU A(i).quadrature.B(j)=min[A(i), B(j)] (5)
The formula (2) represents an absolute value of a difference between A(i) and B(i), the formula (3) represents accumulated product, the formula (4) represents a larger one of A(i) and B(j), and the formula (5) represents a smaller one. By the above definition, V.sub.1 (k) and V.sub.2 (k) can be considered as correlation amounts in a broad sense. From the formula (1), V.sub.1 (k) represents the correlation amount at a displacement (k-1) and V.sub.2 (k) represents the correlation amount at a displacement (k+1). Accordingly, an evaluation amount V(k) which is the difference between V.sub.1 (k) and V.sub.2 (k) represents a change of correlation amount of the image signals A(i) and B(i) at a relative displacement k. Since a change is zero at the peak of the correlation amount, it is assumed that the peak of the correlation amount exists in a section [k, k+1] represented by EQU V(k).multidot.V(k+1)&lt;0 (6)
and V(k) and V(k+1) are interpolated to detect the deviations of the image signals A(i) and B(i). FIG. 6 shows the image signals A(i) and B(i) for the two images formed when the number of photo-electric elements is 16 (N=16). There is a deviation of P. FIG. 7 shows the evaluation amount V(k) of the formula (2) when the relative displacement k is changed within a range of -N/2.ltoreq.k.ltoreq.N/2. As described above, V(k) and V(k+1) which meet V(k).multidot.V(k+1)&lt;0 are linearly interporated to detect the deviation P. FIG. 8 shows a relationship between the image signals A(i) and B(i) when the evaluation amount V(k) is calculated while the relative displacement k changes in a range of -3.ltoreq.k.ltoreq.3. Hatched areas show the photo-electric conversion elements which are subject of the correlation.
Because the operation of the evaluation amount V(k) is done by correlation, the number of steps of operation increases as a function of N.sup.2, where N is the number of photo-electric conversion elements. As a result, as the number N increases, the correlation operation time increases and the focus state detection time increases.